I'm trying to clean up the brakes from a 91 Integra. I took it apart and cleaned one of the front calipers but now I can't seem to get the piston back in. I tried using a piece of wood and a c-clamp to press it back in but it just doesn't want to go. How hard should it be to get in? There was some rust on the inside of the caliper, do I need to do some sanding or something to get more of that out?